This invention relates to the charging of materials by triboelectrification, especially the fine polymer powders used as toners in electrophotographic systems.
The charging of materials by triboelectrification has been applied to a number of industrial products for some time. Since the invention of the electrophotographic technique used for copiers and non-impact printers by Carlson in 1938 (see U.S. Pat. No. 2,297,691 (Carlson, 1942)), the field has developed into a large commercial market. Recently, electrophotography has required higher resolution images than was necessary previously, for application in colour image systems (see E. Czech, W. Ostertag, SPIE 1253, Hard Copy and Printing Products, 64, (1990)). For this purpose it is very important to control accurately the electrical charge of the fine polymer powders used as toners in electrophotographic systems (see S. Kume, The Institute of Electrostatics of Japan 10(5), 306 (1986)).
Toners are fine polymer particles typically about 10 .mu.m in diameter mixed with various additives and usually include a coloured dye. In a two-component development system, the toner particles are charged by making contact with larger metal beads known as carriers [see L. B. Schein, "Electrophotography and Development Physics", ISBN 3-see 540-18902-5, Springer Verlag (1988)]. The toner is transferred to the photoreceptor due to an attractive electric field to form a real image (development). For a high quality image, it is important to control the charge-to-mass ratio (Q/M) of the toner within predetermined limits. The Q/M varies with changes in environmental conditions and surface properties of the toner (see N. Matsui, K. Oka and Y. Inaba, J. Electrophotographics 30(3), 282 (1991)). Much work has been done to investigate the triboelectrification of fine polymer powders (see J. Henniker, Nature 196, 474 (1962); C. B. Duke and T. J. Fabish, J. Appl. Phys. 49, 315 (1978); and L. B. Schein and M. Latta, J. Appl. Phys. 69 (10), 6817 (1991)), but the mechanisms are still not fully understood. Some investigations suggest that the nature of the chemical species on the surface is the most important aspect for controlling the triboelectric charge of the particle (see I. Shinohara, F. Yamamoto, H. Anzai, and S. Endo, J. Electrost. 2, 99 (1976), and H. W. Gibson, Polymer 25, 3 (1984)).